1. Technical Field
The present invention relates to a method of manufacturing a metallic bellows and to an apparatus for manufacturing a metallic bellows. It relates, in particular, to a method of, and an apparatus for, manufacturing a metallic bellows which is used in a flexible tube to be interposed in an exhaust pipe for a vehicle such as a motor vehicle.
2. Related Art
Generally, a flexible tube using a metallic bellows is interposed on the way to outlet of an exhaust pipe of a vehicle such as a motor vehicle. This flexible tube serves the purpose of absorbing the vibrations from an engine of the motor vehicle, thereby preventing the vibrations from being transmitted to such a part as a muffler on a downstream side of the exhaust system. This kind of metallic bellows is conventionally known to be manufactured in the following method.
Specifically, both end portions of a raw pipe are fitted into a pair of upper and lower end dies. A plurality of intermediate annular dies are interposed at an equal interval from one another between the end dies. Thereafter, in a state in which both ends of the raw pipe are hermetically sealed, there is performed an operation of forming or machining known by the name of bulging which makes use of fluid pressure such as hydraulic pressure or a suitable deformable pressure medium such as urethane rubber, and the like. In this manner, the raw pipe is expanded from the inner side and is also swelled at the portions which form crest (or mountain) portions. Then, the end dies are respectively moved toward each other to thereby perform compression-forming until the intermediate annular dies are brought into close contact with one another. A metallic bellows is thus manufactured. JP-A-1249/1997 is an example of relevant prior art; see for example the description of the prior art.
In the above-described example, the pressure is applied from the inside at the time of bulging to thereby radially swell the portions which form the crest portions of the bellows. As a consequence, there is a problem in that the plate thickness at the crest portions becomes smaller than that at trough (valley) portions, thereby resulting in nonuniform plate thicknesses at the bellows section. Further, there is also another problem in that the shapes of the crest portions and the trough portions are unstable (i.e., the shapes of the U-shaped end portions of the crest portions and the trough portions become irregular).
The spring constant of a metallic bellows can be made smaller by reducing the plate thicknesses at the crest portions and the trough portions of the bellows section. However, in the manufacturing method as described above in which there occurs a difference in plate thicknesses between the crest portions and the trough portions of the bellows section, there is a limit to an attempt to minimize the plate thicknesses. In particular, in an example in which the raw pipe is manufactured with a sheet of metallic plate which is die-cut (or stamped) into a predetermined shape and the free ends thereof are then joined together for welding them together, the welded joint is likely to be damaged. Therefore, it was difficult to reduce the thicknesses at the crest portions and the trough portions of the bellows section. It may then be considered to make the spring constant smaller by increasing the axial length of the bellows section or by enlarging the distances between the respective adjoining crest portions and the trough portions at the bellows section. However, this solution has a problem in that the flexible tube becomes larger in size and weight.